AAS 204th Meeting, June 2004
Session 76 Data Handling, Computation
Poster, Thursday, June 3, 2004, 9:20am-4:00pm, Ballroom

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[76.11] Observing Redshifted HI in the 962-1213 MHz Aircraft Navigation Band

J. R. Fisher (NRAO), Q. Zhang, Y. Zheng, S. G. Wilson (Univ. of Virginia), R. F. Bradley (NRAO)

Measurements of redshifted absorption lines of neutral hydrogen below 1420 MHz and the OH molecule below 1660 MHz and OH megamaser emission are important to studies of the evolution of the gaseous content of galaxies. These spectral lines fall in a part of the radio spectrum that is heavily occupied by man-made signals. This poster paper reports on studies of signal characteristics in the 962-1213 MHz aircraft navigation band and signal processing algorithms for removing these signals from astronomical data. This frequency range corresponds to HI redshifts of z = 0.17 to 0.0.48 and OH redshifts of z = 0.37 to 0.73.

Most of this band is allocated to Distance Measuring Equipment (DME) where aircraft determine their distances to ground stations by measuring the delay between their transmitted pulses and reply pulses from the ground. The aircraft transmit in 1-MHz channels between 1025 and 1150 MHz, and the ground reply frequencies are in the 962-1024 and 1151-1213 MHz bands. From Green Bank signals can be seen in every aircraft channel for which there are assignments in the eastern U.S., sometimes from tens of aircraft on each frequency. Ground station signals are much weaker or not seen at all. Because the pulse duty cycle for one aircraft is less than about 0.01%, individual pulses can be detected and removed from the data stream before entering an FFT or correlation spectrometer. An example is shown of the detection of a 15 milli-Jansky absorption line with the GBT at 1147.5 MHz near one of the most heavily occupied DME channels. Pulse density limits are given for effective signal removal from the astronomical spectra.

The author(s) of this abstract have provided an email address for comments about the abstract: rfisher@nrao.edu

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